Did the doc refer u. There are agencies like Heep Hong and Caritas that do therapy for a reasonable fee. Private agencies charge more, but still less than doctors or psychiatrists.
There are many types of therapy: 1 to 1, sensory stimulation, muscles ex, music, art, speech. Ask the doc what he needs and start asap.
Govt assessment centres can refer you to kindergartens with help for such kids. If he is already in primary school, tell the school social worker to organize some help. All govt and subsidized schools are required to help such students.